Since metal nanoparticles possess huge surface areas and very high chemical activities, they have now become a popular topic of investigation in various fields. For example, silver nanoparticles have shown outstanding performance in aspects such as conductivity, optical and thermal properties, and catalyzing reaction. Research has also demonstrated that silver nanoparticles are capable of sterilizing bacteria and viruses without harming the body and the environment. Thus, they are regarded as sterilizing materials, which offer superior safety. In light of these desirable features, the method for preparing and manufacturing silver nanoparticles has attracted much attention from various industries.
There are several conventional methods for manufacturing metal nanoparticles, including chemical reduction, microemulsion, photovoltaic method, pyrolysis, mechanical grinding, and physical vapour deposition. In which, the most common one is the chemical reduction method. The advantages of using the chemical reduction method include its low capital cost and its simple operation. It is also applicable to non-metal materials and metal alloy materials. The industrial chemical reduction method is often performed in a batch reactor. Even though the procedure is simple, the size of the end product is often too large and widely distributed. Furthermore, the production rate using this method is also low.
The published methods for manufacturing metal nanoparticles to date are numerous. For example, Wallen et al. has published an article entitled ‘Completely “Green” Synthesis and Stabilization of Metal Nanoparticles’ (Journal of American Chemistry Society, 2003, 125: 13940-13941), which described a method of manufacturing silver nanoparticles using glucose and silver nitrate as reactants and starch as protecting agent. The process requires the temperature to be set at 40° C. and a reaction time of 20 hours. Ren et al. published an article entitled ‘Synthesis of nanosized silver particles by chemical reduction method’ (Materials Chemistry and Physics, 2000, 64: 241-246), which described a method of manufacturing silver nanoparticles using silver nitrate, formaldehyde and polyvinylpyrrolidone (PVP) with a reaction time of 60 minutes. On the other hand, the method disclosed in the U.S. Pat. No. 6,929,675 reacts (Cu(C6H2(CH3)3)5, (Ag(C6H2(CH3)3)4 or (Au(C6H2(CH3)3)5 with amine at a temperature slightly over 100° C. for at least 1 minute to manufacture nanoparticles of copper, silver or gold. A method disclosed in the Taiwan Patent Open No. 200426114 (whose application number is 92113427) manufactures silver nanoparticles by reacting silver nitrate solution and trisodium citrate solution at 65-150° C. for 0.5-60 minutes.
The conventional methods for manufacturing metal nanoparticles are generally time-consuming (the reaction time is at least 1 minute and often in the hours). The conventional methods also require the use of a huge quantity of chemicals such as formaldehyde that is detrimental to the environment. Therefore, in order to protect the environment, industry has focused on developing a method for manufacturing metal nanoparticles that incorporates both features of fast reaction and minimal impact on the environment.
In order to overcome the drawbacks of the inability of conventional method to achieve fast reaction and minimal environment impact simultaneously, a new method for manufacturing metal nanoparticles is desired.